Deciphering the rules underlying xenogeneic silencing and counter-silencing of Lsr2-like proteins

Lsr2-like nucleoid-associated proteins play an important role as xenogeneic silencers (XS) of horizontally acquired genomic regions in actinobacteria. In this study, we systematically analyzed the in vivo constraints underlying silencing and counter-silencing of the Lsr2-like protein CgpS in Corynebacterium glutamicum. Genome-wide analysis revealed binding of CgpS to regions featuring a distinct drop in GC-profile close to the transcription start site (TSS), but also identified an overrepresented motif with multiple A/T steps at the nucleation site of the nucleoprotein complex. Binding of specific transcription factors (TFs) may oppose XS activity leading to counter-silencing. Following a synthetic counter-silencing approach, target gene activation was realized by inserting operator sites of an effector-responsive TF within various CgpS target promoters resulting in an increased promoter activity upon TF binding. Analysis of reporter constructs revealed maximal counter-silencing when the TF operator site was inserted at the position of maximal CgpS coverage. This principle was implemented in a synthetic toggle switch, which features a robust and reversible response to effector availability highlighting the potential for biotechnological applications. Altogether, our results provide comprehensive insights into how Lsr2 silencing and counter-silencing shapes evolutionary network expansion in this medically- and biotechnologically-relevant bacterial phylum.\n\nIMPORTANCEIn actinobacteria, Lsr2-like nucleoid-associated proteins function as xenogeneic silencers (XS) of horizontally acquired genomic regions, including viral elements, virulence gene clusters in Mycobacterium tuberculosis, and genes involved in cryptic specialized metabolism in Streptomyces species. Consequently, a detailed mechanistic understanding of Lsr2 binding in vivo is relevant as a potential drug target and for the identification novel bioactive compounds. Here, we followed an in vivo approach to investigate the rules underlying xenogeneic silencing and counter-silencing of the Lsr2-like XS CgpS from Corynebacterium glutamicum. Our results demonstrated that CgpS distinguishes between self and foreign by recognizing a distinct drop in GC-profile in combination with a short, sequence specific motif at the nucleation site. Following a synthetic counter-silencer approach, we studied the potential and constraints of transcription factors to counteract CgpS silencing thereby facilitating the integration of new genetic traits into host regulatory networks.